As water in streams and rivers flow, pollutants and contaminants that come from urban, agricultural, and industrial runoff are carried downstream. But how and why are scientists finding contaminants upstream of industries, farms, and developments? The culprit: salmon.

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Research by University of Notre Dame stream ecologist Gary Lamberti and his laboratory has revealed that salmon, as they travel upstream to spawn and die, carry industrial pollutants into Great Lakes streams and tributaries.

The team advises, if you plan on catching and eating fish from a Lake Michigan tributary with a strong salmon run, the fish may be contaminated by pollutants.

When salmon travel upstream to spawn and die, they leave behind the contaminants they have absorbed in their fatty tissues over their lifetime in the form of salmon eggs and carcass tissue that will be eaten by other organisms.

"All the Great Lakes have some level of pollution," says Lamberti, "especially near cities — Chicago, Detroit, Cleveland. There are far fewer pollutants now than over the past century, but many are persistent. There are hot spots, and Lake Michigan has a lot of them — heavy metals, mercury, organic pollutants like PCBs." PCBs come from fluids in older electrical transformers. Also present is DDE, a breakdown product of the banned insecticide DDT.

Salmon acquire pollutants through the lake food chain. When they are young, they feed on invertebrates. As they grow, salmon consume more and more fish, which have also picked up pollutants through the invertebrates they eat, which essentially come from algae and bacteria.

The salmon magnify the pollutants as they move up the food chain. "Salmon are longer lived, eat more, and the pollutants are then bio-concentrated."

Lamberti's research analyzed the water and tissue of fish upstream where salmon spawn and die along with the water and fish downstream.

"The upstream section of the same river was not contaminated. Below the salmon, the river had measurable levels of contaminants. There's no other way for the contaminants to get there but the salmon. Water doesn’t flow uphill."

Another point of interest is that the salmon are not native to the Great Lakes. Lamberti explains, "They were introduced to control alewives — another non-native fish species."

Although salmon fed on and contained the alewives population, there have been unintended consequences because of the industrial pollution of the Great Lakes.

Although salmon are an economic benefit to the Great Lakes and perform important ecological functions, they are also unintentionally carrying pollutants upstream.

The research was recently published in the journal Environmental Science and Technology.